The invention relates to a method for compensating for magnetic noise fields in spatial a volume, comprising the following steps:
determining characteristic data or parameters describing a magnetic noise field outside said spatial volume;
defining, on a theoretical basis, i.e. according to the laws of electromagnetism, or on an empirical basis, the correlation between the noise field outside the spatial volume and a corresponding noise field inside said spatial volume, or calculating the noise field inside the spatial volume, from the measurement/s of the magnetic field outside the spatial volume;
from the inside magnetic field, as determined theoretically or empirically, generating a magnetic compensation field, particularly for neutralizing the noise field in said spatial volume.
In the following description and claims, “noise fields” shall be intended as magnetic field fluctuations induced in a spatial volume, particularly the imaging cavity of Magnetic Resonance imaging apparatuses, which fluctuations are caused by magnetic fields outside said spatial volume or the imaging cavity of the MRI apparatus.
The term characteristic parameters describing a magnetic noise field refer primarily to the strength of the magnetic noise field and may also include the direction of the magnetic noise field which can be measured by a three axial magnetometer or similar instruments.
Currently, the wide use of electric power causes the generation of considerable magnetic fields, which pervade the environment. These magnetic fields may have considerable strengths and affect or alter the operation of electric or electronic equipment which use magnetic fields. Such equipment may be of any type, e.g. measuring instruments, diagnostic or therapeutic imaging apparatuses, and the like.
Currently, the above noise may be caused by two types of magnetic fields, which are differentiated on the basis of frequency and of the amplitude of strength of the magnetic field. A first so-called low-frequency type includes magnetic fields with frequencies ranging from less than 1 Hz to a few units of Hz. These types of low-frequency noise fields are typically generated by the passage of vehicles or the like. Each vehicle generates, in first approximation, a magnetic dipole with a predetermined strength and a predetermined position and the dipoles are oriented in the direction of the earth's magnetic field flow lines.
A second type includes noise fields generated by sources of mains AC, which have frequencies of about 50 to 60 Hz. Besides home or static sources, electric vehicles shall be also considered, such as trains, tramcars, subway trains, trolley-buses, etc., due to the considerable powers absorbed and the strengths of the fields generated thereby.
A third category might include noise fields with frequencies of the order of a fraction of the mains frequency, i.e. of about 10 to 20 Hz. Particularly, some railway electrification lines use, for instance, one-third of the mains frequency, that is a frequency of the order of 16 Hz.
Other noise fields consist of fast, i.e. high-frequency transients. In spatial volumes containing significant electrically conductive masses, e.g. shielding magnetic structures or Faraday cages, these fast transient fluctuations of the magnetic noise field induce noise currents, which modify the noise fields inside the imaging cavity. These fast magnetic field transients may be also combined with other noise types.
Currently, the methods of compensation for magnetic fields like the ones described hereinbefore use one or more sensors for determining the magnetic field amplitude and frequency.
The noise field inside the spatial volume is compensated by generating an inverse magnetic field to the magnetic noise field inside the spatial volume.
This is actuated by providing a magnetic field compensator generating the compensation magnetic field inside the spatial volume.
Methods and systems for compensating magnetic noise fields in spatial volume, and particularly in the gantry of an MRI apparatus are known from document EP1353192B1 and from document U.S. Pat. No. 7,504,825 assigned to the same applicant of the present application.
In this document, a compensation method for the magnetic noise inside the spatial volume, for example the gantry of the MRI apparatus is determined according to a transfer function of the magnetic noise field measured outside said spatial volume and by generating a magnetic compensation field which compensates the magnetic noise field determined according to said transfer function.
In said prior art solution, the system for carrying out the compensation method is an active open loop compensation.
The system for carrying out the magnetic compensation comprises a magnetic compensation field, generator which generates a magnetic compensation field permeating limited to the spatial volume, for example a magnetic compensation field generator which is housed within the magnetic structure of an MRI scanner.
FIG. 1 illustrates a block diagram illustrating the system according to the prior art according to an embodiment in which said system is applied to an MRI apparatrus.
Generally, MRI apparatus are housed inside a Faraday cage 110 for shielding electromagnetic noise.
A magnet structure which is summarized as a box 150 and generally comprising a static magnetic field generator in a spatial volume, gradient coils for generating gradient magnetic fields inside said spatial volume, RF-coils for generating RF excitation signals of the nuclear spins due to the physical capacity of atomic nuclei of absorbing and re-emitting electromagnetic radiation, RF-antenna for collecting the RF signals emitted by the transition of the nuclear magnetic spins from the excited condition in which the nuclei has absorbed electromagnetic energy into the relaxed condition in which the nuclei has re-emitted the absorbed electromagnetic energy.
Different kinds of static magnetic field generators are known which are principally:
Superconductive magnets in which a superconductive coil is energized to produce the static magnetic field,
Resistive magnetic field generators, in which electric conductive coils are energized to generate a magnetic field, and
Permanent magnets in which permanently magnetized material is used to generate the static magnetic field. Magnets combining said different technologies are also provided.
A generic structure of an MRI apparatus further comprises:
a magnet driving and control section which is responsible for driving the resistive or superconductive magnets such that a certain static magnetic field is generated or which controls the temperature of the magnetized material in case of permanent magnets;
a compensation magnetic field generator and a compensation magnetic field controller for driving said compensation magnetic field generator in such a way as to compensate magnetic noise fields;
a gradient coils driving unit which provides driving of the gradient coils in a synchronized way with the generation and transmission of the RF-excitation signals;
a RF generation unit for generating the RF excitation signals to be transmitted by the RF excitation coils to the target region of a body under examination and which region is coincident or contained inside a spatial volume, i.e. an imaging volume permeated by the static magnetic field and the gradient magnetic fields;
a processing unit of the RF signals acquired by the RF antenna for transforming said RF signals in image data;
a display control unit for processing the image data in order to display these data according to different display modes on a display monitor.
The static magnetic field provides for the orientation of the magnetic nuclear spins along a uniform direction which is parallel to the direction of the static magnetic field. A high spatial homogeneity of the static magnetic field is necessary in order that the RF data can be processed to image data reproducing the internal structure of a target region being examined without introducing artifacts.
Magnetic noise fields which permeates the spatial volume 160 add to the static magnetic field compromising the homogeneity of the static magnetic field.
As illustrated by FIG. 1 the magnetic noise fields 100 penetrate through the Fraday cage 110 and permeate into the spatial volume 160, in which they add to the static magnetic field B0.
A magnetic compensation field is applied by a magnetic compensation field generator 170 provided inside the magnetic structure 150. This magnetic compensation field generator is in the form of one or more resistive coils which are combined or simply overlaid to the static magnetic field generator magnets and to the gradient magnets and to the other units provided in combination therewith.
The magnetic compensation field generator 170 is driven by a driving and control chain which comprises a magnetic noise field sensing module 120, a compensation field control module 130 which processes the output signals of the magnetic noise field sensing module 120 and which compensation field control module 130 generates driving signals of a power module 140. This power module 140 generates power driving signals of the magnetic field compensation generator 170 according to the control signals of the control module 130. The control module 130 drives the power module 140 so that a magnetic compensation field is generated and superimposed to the static magnetic field and to the noise field inside the spatial volume such that the magnetic noise field is compensated in such a way that the remaining noise field inside the spatial volume 160 approaches to zero.
The method according to said prior art method and system has limitations in effectively compensating all kinds of noises.
The compensator according to the prior art is not able to effectively manage the contemporaneity of on one hand rapid disturbances and on the other hand periodic disturbances (16, 66, 50 to 60 Hz) or, in the case of periodic interferences, also the amplitude modulations of these periodic interferences.
Furthermore, due to the fact that the compensation magnetic field generating coils have limitation in the homogeneity of the field generated by them when the intensity of the magnetic field to be compensated is relatively high, the method and system according to the prior art do not manage noise on the magnet which is higher than 10 mGpp.
FIG. 2 shows a Magnetic Resonance imaging apparatus which integrates an open loop magnetic noise field compensator device according to the invention.
In this case, the spatial volume wherein the noise fields which are to be compensated for coincide with the cavity of the Magnetic Resonance imaging apparatus which is to accommodate the patient body or a part thereof and is indicated as V.
FIG. 2 describes an embodiment which includes at least one probe S1 and allows the use of multiple probes as shown by the probes S2, S3, S4, S5 and S6, which are shown by dashed and dotted lines.
A dashed line and the numeral 200 denote the control and processing electronics of a Magnetic Resonance imaging apparatus, whereas the blocks included therein are additional functional units, or have functions accomplished by appropriately programmed or controlled units of the Magnetic Resonance imaging apparatus.
These control and processing electronics drive the magnetic structure of the MRI system comprising a magnet 210 for generating a static magnetic field B0 within a cavity at least partially delimited by the magnet and within an imaging volume V in said cavity.
The Magnetic Resonance imaging apparatus includes static magnetic field B0 generators, which are denoted with numeral 211.
These static magnetic field generators can be superconductive coils, resistive coils permanent magnets, or combinations thereof.
In an embodiment of the present invention for simplicity the static magnetic field generator comprises two opposite magnetic poles enclosing a cavity for accommodating the body under examination, or a part thereof.
Control electronics of the static magnetic field generator are not illustrated in detail since it is part of the common general knowledge of the skilled person.
Furthermore, the magnet structure of the MRI apparatus comprises at least one magnetic noise field compensation coil 212 for generating inside the cavity and the imaging volume V a magnetic noise compensation field which neutralizes at least partially the effect of the magnetic noise fields on the static magnetic field B0.
The system further comprises at least one exciting pulse transmission coil 213 for transmitting to the body under examination one or more sequences of RF pulses for NMR excitation which coil is controlled by a RF excitation pulse generator 201.
The magnet structure comprises magnetic gradient fields generating coils 214 for generating during scanning, a magnetic field with a predetermined variation along each of three spatial directions (x, y, z) having the function of univocally encoding the nuclear spins and thus allowing to relate the NMR signal contributions of the RF signals received by the RF antenna 215 to a position in space for reconstructing an image. The magnetic gradient fields generating coils 214 are driven by a gradient field coils driving unit 202.
The RF NMR signals are processed by a processing unit converting the RF data into image data indicated by 204 and to an image generation unit 205. The reconstructed images can be displayed on a display 220 or stored in memories 230 which can be alternatively or in combination internal memories of the MRI apparatus or memories residing in an external storage servers or in cloud servers. Optionally the images can be stored in portable memory devices 240 such as CD rom, DVD rom CD RAM, DVD RAM, memory sticks, portable hard disks, or similar devices.
A user interface 209 providing alternatively or in combination different user input devices is provided. The user interface may be alternatively or in combination a graphical user interface, a vocal user interface, a keyboard, a mouse or a similar device, a input port of command strings which has been generated by a remote device.
The probe S1 and/or other possible probes S2, S3, S4, S5, S6 are connected to the input of a processing unit 203 for determining characteristic parameters of the magnetic noise field outside the cavity V of the Magnetic Resonance imaging apparatus and this characteristic data is fed to a compensation coils controller 207.
According to an embodiment, the compensation coils controller 207 is provided with a processor unit configured to calculate from theoretical or empiric functions the characteristic parameters of the magnetic noise field inside the cavity from the characteristic parameters of the noise field outside the cavity. The processor unit of the compensation coils controller is also configured to determine the magnetic noise compensation field to be generated inside the cavity for neutralizing the magnetic noise field inside said cavity. This magnetic noise compensation field is determined as a function of the characteristic parameters describing the magnetic noise field inside said cavity which has been determined from the measured characteristic parameters of the magnetic noise field outside the cavity.
The compensation coil controller 207 controls a compensation field generator 208 which provides the driving power signals for feeding the compensation coils 212.
According to an embodiment the control electronics of the MRI apparatus can be entirely or at least in part in the in the form of software units, consisting of programs for controlling programmable hardware of the Magnetic Resonance imaging apparatus, such as a PC or a central processing and control unit. Optionally the control electronics of the MRI apparatus can be entirely or in part dedicated hardware in which the functional logic is incorporated in the hardware.
The compensation coils 212, already resident in the Magnetic Resonance imaging apparatus, are adapted to compensate for substantially uniform and homogeneous static field fluctuations, induced by outside magnetic fields, in the imaging cavity V. Nevertheless, some fluctuations may show some spatial variability within the cavity, i.e., spatial non homogeneities.
In this case, by suitably controlling gradient coils 214, compensation fields may be also generated for these field fluctuations, induced by noise fields, which have non-uniformities and non-homogeneities in space.